Structure and species richness in wetland continua on sandy soils in subtropical and tropical Australia



Net photosynthetic fixation of wetland plant communities is confined to the period of the year when the surface soil is not waterlogged and is thus well aerated. In the open-structured vegetation continuum across freshwater wetlands on sandy soils in subtropical and tropical Australia, the sum of the foliage projective covers (FPCs) of the overstorey and understorey strata remains constant, while that of the overstorey decreases to zero as seasonal waterlogging (and anaerobic conditions) in the surface root systems increases. Density and height of the overstorey trees – of only one or two species – and species richness (number of species per hectare) in the understorey decreases along this waterlogging gradient. Melaleuca paperbark trees, possessing surface roots with cortical aerenchyma, may form a closed-forest at the edge of the wetland continuum wherever there is a continuous flow of aerated water. As global warming progresses, an increase in air temperature in the atmosphere flowing over and through the wetland continuum during the short period of annual foliage-growth will affect the combined FPCs of overstorey and understorey strata, as well as the leaf-specific weights of all leaves throughout the plant community. With a reduction in net photosynthetic fixation, species richness of the plant community will slowly decline.